This invention relates generally to read-out circuits for rotary shaft or other position encoders using coarse and fine binary digital data generators, and more particularly to an improved read-out circuit, or system employing such circuit, of the type that combines two digital measurements of different resolutions into a single digital measurement.
The measurement and interpretation of angular shaft position, for example, has been accomplished using either plural rotary discs driven in a predetermined ratio and carrying code patterns having coarse and fine angular resolutions or using a single disc with coaxial patterns of different resolutions. The code patterns are detected, coverted to overlapping binary numbers representative of the coarse and fine resolution disc positions, the binary numbers interpreted by logic means to provide a composite binary digital number that can be used in that form or be converted to an analog angular measurement of the shaft position. U.S. Pat. Nos. 2,630,552, 2,779,539, 2,885,613, and 3,534,360 provide examples of such systems with certain variations.
One of the more troublesome and prevalent problems in such systems is found to lie in the manner of justification of, or correction to, the coarse binary readout in order to accommodate differences between the overlapping portions of the binary readouts of the coarse and fine resolution patterns so as to provide a composite number that represents the shaft angular position with the accuracy desired. In at least one known prior art system, the problem has manifested itself in a non-monotonic increase in the output of the system for monotonically increasing input angles with the result that, when the system forms part of a position measuring system, such as connected with operation of a submersible vehicle control surface for example, there has been an undesirable amount of chatter developed at many position angles.
In that system using a stator and rotor disc set and intended to provide angular measurement of shaft position with an accuracy of 0.005 degrees over a full 360 degrees the discs are embedded with a single coarse pattern that covers the entire 360 degrees and 128 fine patterns, each covering 2.8125 degress. Movement of the coarse pattern produces analog voltages that are converted and read out as an eleven bit binary digital number with each successive bit position in the number representing a larger angular increment or bit value. The fine pattern read out is a nine bit binary digital number with each successive bit position in the number also representing a larger angular increment or bit value. These digital numbers overlap in bit values by 4 bits which, ideally, would be identical and a composite binary number accurately representative of the shaft position would consist of the upper seven most significant bits of the coarse pattern number and the nine bits of the fine pattern number.
The comparison logic operates on three of the four overlapping binary bits of the coarse and fine binary numbers to determine the need to add one count to the non-overlapping bits of the coarse pattern number, to subtract one count therefrom, or to leave that number as is. Because the comparison logic is capable of using only three overlapping bits, the determinable difference therebetween is limited to a decimal 7, giving rise to inconsistent corrective actions for a given decimal difference. Moreover, the angular difference between the coarse and fine patterns that can properly be protected against in that system is 2 counts of the 0.352 degree bit or 0.704 degrees.